Method for correcting scanner non-uniformity

1. A method for identifying and correcting for spatial variation within scanning bars, the method comprising: performing a plurality of diagnostic scans of at least one sheet, wherein said at least one sheet is scanned in a plurality of orientations and translations, and wherein each of said orientations can be identified; registering said plurality of diagnostic scans with at least one of said diagnostic scans to produce registered scans; and averaging said registered scans to create a master scan corresponding to said at least one sheet, wherein said master scan is indicative of the spatial variation of said at least one sheet.

2. The method for identifying and correcting for spatial variation within scanning bars according to claim 1 , wherein registering said plurality of diagnostic scans comprises: selecting one of said diagnostic scans as a preliminary master scan; determining a transformation that minimizes the error between the locations of image features in said master scan and a second scan; and applying said transformation to the raster of said second scan.

3. The method for identifying and correcting for spatial variation within scanning bars according to claim 2 , wherein said image features comprise at least one member selected from the group consisting of fiducial marks, image edges, and centroids of image regions.

4. The method for identifying and correcting for spatial variation within scanning bars according to claim 2 , wherein said locations include the centers of fiducial marks in said master scan and the centers of the corresponding fiducials in said second scan.

5. The method for identifying and correcting for spatial variation within scanning bars according to claim 1 , further comprising: registering said master scan with each of said marking scans to produce a registered master scan; and comparing said registered master scan with said each of said marking scans to produce an error image of the scanner and detecting variations in said error image.

6. The method for identifying and correcting for spatial variation within scanning bars according to claim 1 , further comprising correcting for integrating cavity effect, wherein said integrating cavity effect causes the edge regions of each said diagnostic scan to differ from the center of said diagnostic scan.

7. The method for identifying and correcting for spatial variation within scanning bars according to claim 6 , wherein correcting for integrating cavity effect comprises: breaking said diagnostic scans into tiles; calculating an integrating cavity effect correction on a tile-by-tile basis; scaling the correction up utilizing linear interpolation; and applying said correction to the full resolution image of each of said diagnostic scans.

8. The method for identifying and correcting for spatial variation within scanning bars according to claim 6 , wherein correcting for integrating cavity effect comprises: calculating an integrating cavity effect correction on a pixel-by-pixel basis; and applying said correction to the full resolution image of each of said diagnostic scans.

9. The method for identifying and correcting for spatial variation within scanning bars according to claim 1 , wherein averaging said registered scans to create a master scan comprises computing the mean and standard deviation of each pixel across said diagnostic scans and disregarding a scan for an individual pixel that exceeds a predetermined number of standard deviations from the mean for said individual pixel.

10. The method for identifying and correcting for spatial variation within scanning bars according to claim 1 , wherein creating said master scan comprises constructing a new image from said computed mean for each pixel of said scanned images.

11. The method for identifying and correcting for spatial variation within scanning bars according to claim 5 , wherein comparing said master scan with each of said diagnostic scans comprising subtracting said master image from at least one of said scanned images.

12. The method for identifying and correcting for spatial variation within scanning bars according to claim 11 , wherein subtracting said master image from said at least one scanned image comprises applying the inverse of said transformation developed for registering said diagnostic scans to said preliminary master.

13. The method for identifying and correcting for spatial variation within scanning bars according to claim 5 , wherein detecting variations comprises: computing the average of the error image, down each column of pixels, to produce a single average error scanline across the fast scan direction for each of said scanned images; and applying further analysis to separate out scan-to-scan variation from trends that are consistent across scans.

14. The method for identifying and correcting for spatial variation within scanning bars according to claim 5 , wherein detecting variations comprises: computing the average of the error image, .across each row of pixels, to produce a single average error column for the slow scan direction for each of said scanned images; and applying further analysis to separate out scan-to-scan variation from trends that are consistent across scans.

15. The method for identifying and correcting for spatial variation within scanning bars according to claim 5 , further comprising correcting variations in said scanned image or in at least one subsequent scanned image.

16. The method for identifying and correcting for spatial variation within scanning bars according to claim 15 , wherein correcting variations comprises at least one member selected from the group consisting of subtracting error values from each pixel, subtracting interpolated error values from each pixel, evaluating a model to produce a correction amount, and evaluating a model to produce a corrected color value.

17. A method for identifying and correcting for spatial variation within scanning bars, the method comprising: performing a plurality of diagnostic scans of at least one sheet, wherein said at least one sheet is scanned in a plurality of orientations and translations, and wherein each of said orientations can be identified; registering said plurality of diagnostic scans with at least one of said diagnostic scans to produce registered scans; averaging said registered scans to create a master scan corresponding to said at least one sheet, wherein said master scan is indicative of the spatial variation of said at least one sheet; registering said master scan with each of said marking scans to produce a registered master scan; comparing said registered master scan with said each of said marking scans to produce an error image of the scanner and detecting variations in said error image; and correcting variations in said scanned image or in at least one subsequent scanned image, wherein correcting variations in said scanned image or said at least one subsequent scanned image comprises: computing the standard deviation of each said average of the error image and dividing by the square root of the number of pixels contributing to obtain a standard error of the mean; determining the percentile rank of said errors computed; multiplying said percentile rank by said standard error of the mean; and subtracting said standard error of the mean from said computed average error.

18. The method for identifying and correcting for spatial variation within scanning bars according to claim 1 , wherein said at least one sheet comprises at least one member selected from the group consisting of a single white sheet, a sheet representing the light end of the color scale and a sheet representing the dark end of the color scale, or a plurality of sheets representing a plurality of the parts of the color space.

19. The method for identifying and correcting for spatial variation within scanning bars according to claim 15 , wherein correcting variations in said scanned image comprises applying either spline interpolation or tetrahedral interpolation to each pixel.

20. The method for identifying and correcting for spatial variation within scanning bars according to claim 1 , wherein performing a plurality of diagnostic scans comprises: scanning at least one sheet in a first location on a scanner platen; scanning said at least one sheet in a second location on said scanner platen; rotating said at least one sheet 90 degrees and scanning said at least one sheet in said first location on said scanner platen; scanning said rotated at least one sheet in said second location on said scanner platen; and repeating rotating said at least one sheet 90 degrees and scanning said at least one sheet in each of said first location and said second location on said scanner platen until scans of all orientations of said sheet have been performed.

21. A system for identifying and correcting for spatial variation within scanning bars, the system comprising: means for performing a plurality of diagnostic scans of at least one sheet, wherein said at least one sheet is scanned in a plurality of orientations and translations, and wherein each of said orientations can be identified; means for registering said plurality of diagnostic scans with at least one of said diagnostic scans to produce registered scans; and means for averaging said registered scans to create a master scan corresponding to said at least one sheet, wherein said master scan is indicative of the spatial variation of said at least one sheet.